Fixed failure to instantiate nested class group without arguments (lightning#17263).

Author: mauvilsa

CHANGELOG.rst

@@ -37,6 +37,8 @@ Fixed
 - ``dataclass`` from pydantic not working (`#100 comment
   <https://github.com/omni-us/jsonargparse/issues/100#issuecomment-1408413796>`__).
 - ``add_dataclass_arguments`` not forwarding ``sub_configs`` parameter.
+- Failure to instantiate nested class group without arguments (`lightning#17263
+  <https://github.com/Lightning-AI/lightning/issues/17263>`__).
 
 Changed
 ^^^^^^^

jsonargparse/signatures.py

@@ -534,7 +534,6 @@ def group_instantiate_class(group, cfg):
         value = {}
         parent = cfg
         key = group.dest
-        assert '.' not in key
     parent[key] = group.group_class(**value)
 
 

jsonargparse_tests/test_signatures.py

@@ -196,6 +196,26 @@ def __init__(self, a0=None):
         self.assertEqual([], parser.add_class_arguments(NoValidArgs))
 
 
+    def test_instantiate_nested_class_without_args(self):
+        class CallbackWithArgs:
+            def __init__(self, p1: int):
+                self.p1 = p1
+
+        class CallbackWithoutArgs:
+            pass
+
+        parser = ArgumentParser()
+        parser.add_class_arguments(CallbackWithArgs, 'callbacks.first')
+        parser.add_class_arguments(CallbackWithoutArgs, 'callbacks.second')
+
+        cfg = parser.parse_args(['--callbacks.first.p1=2'])
+        self.assertEqual(cfg.callbacks.first, Namespace(p1=2))
+        self.assertNotIn('callbacks.second', cfg)
+        init = parser.instantiate_classes(cfg)
+        self.assertIsInstance(init.callbacks.first, CallbackWithArgs)
+        self.assertIsInstance(init.callbacks.second, CallbackWithoutArgs)
+
+
     def test_add_function_with_dict_int_keys_arg(self):
         def func(a1: Union[int, Dict[int, int]] = 1):
             pass